A. Field of the Invention
The invention relates to the field of database replication. More specifically, the invention relates to a database replication system that utilizes array level processing to execute operations upon a replica database.
B. Description of Related Art
Data replication is the process of maintaining multiple copies of a database object in a distributed database system. Performance improvements can be achieved when data replication is employed, since multiple access locations exist for the access and modification of the replicated data. For example, if multiple copies of a data object are maintained, an application can access the logically “closest” copy of the data object to improve access times and minimize network traffic. In addition, data replication provides greater fault tolerance in the event of a server failure, since the multiple copies of the data object effectively become online backup copies if a failure occurs.
In general, there are two types of propagation methodologies for data replication, referred to as “synchronous” and “asynchronous” replication. Synchronous replication is the propagation of changes to all replicas of a data object within the same transaction as the original change to a copy of that data object. For example, if a change is made to a table at a first replication site by a Transaction A, that change must be replicated to the corresponding tables at all other replication sites before the completion and commitment of Transaction A. Thus, synchronous replication can be considered real-time data replication. In contrast, asynchronous replication can be considered “store-and-forward” data replication, in which changes made to a copy of a data object can be propagated to other replicas of that data object at a later time. The change to the replicas of the modified data object does not have to be performed within the same transaction as the original calling transaction.
Synchronous replication typically results in more overhead than asynchronous replication. More time is required to perform synchronous replication since a transaction cannot complete until all replication sites have finished performing the requested changes to the replicated data object. Moreover, a replication system that uses real-time propagation of replication data is highly dependent upon system and network availability, and mechanisms must be in place to ensure this availability. Thus, asynchronous replication is more generally favored for non-critical data replication activities.
However, the faster an asynchronous replication can store, forward and apply the change to the replica database the more that an asynchronous replication system benefits from the advantages of a synchronous replication system without the associated overhead.
Therefore, there is a need for asynchronous database replication systems that more efficiently use system resources in order to minimize time lag between the moment a change is applied to primary database and the moment a change is applied to a replica database.